This invention relates generally to the field of mass spectrometry and more particularly toward a skimmer having a surface for reduced deposition of unwanted samples, compounds or contaminants.
There are many types of ionization sources that are useful in mass spectrometry (hereinafter referred to as MS). Types of ionization sources include, but are not limited to, electron impact, chemical ionization, plasma, fast ion or atom bombardment, field desorption, laser desorption, plasma desorption, inductively coupled plasma, thermospray and electrospray. Two of the most widely used ionization sources for gaseous analytes are the electron impact (hereinafter referred to as EI) and chemical ionization (hereinafter referred to as CI). Some of these sources have been developed that produce similar ions and results under standard atmospheric pressure conditions. Atmospheric pressure ionization (API) has, therefore, been adopted for use with mass spectrometers. Each of these sources, however, has the common problem of compound deposition due to interaction or bombardment with the internal surfaces of chambers, components and conduits used in transporting ions from the ion source to the detector. More importantly, reactive samples provide an even bigger interaction or deposition problem as they are moved from ion source to detector and contact the inner surfaces of the mass spectrometer. Such reactive species include, but are not limited to trifluoroacetic acid, nitric acid, phosphoric acid, formic acid, ammonium acetate buffers and phosphate buffers. A number of attempts have been made to lower interaction of analyte and solvent with mass spectrometer parts. For example, mass spectrometer parts have been made with electropolished stainless steel surfaces in efforts to reduce the total active surface area. However, mass spectrometers using such parts were found to give variable results and still exhibit deposition of the analyte over time.
Further attempts have been made to focus changes in materials for ionization chambers and ion sources. For instance, U.S. Pat. No. 5,055,678 to Taylor et al. describes the use of a chromium or oxidized chromium surface in a sample analyzing and ionizing apparatus, such as an ion trap or El ionization chamber, to prevent degradation or decomposition of a sample in contact with the surface. This reference also describes coating the inner surface of the ionization chamber with materials known for corrosion resistance or inertness, such as gold, nickel and rhodium. Such surfaces suffer from a variety of drawbacks such as susceptibility to scratching when the metal coating is soft or assembly/diassembly difficulties when the coating has a high coefficient of friction. U.S. Pat. No. 5,796,100 to Palermo discloses a quadrupole ion trap having inner surfaces formed from molybdenum. In addition, U.S. Pat. No. 6,037,587 to Dowell et al. describes a mass spectrometer having a CI source containing a chemical ionization chamber having inner surfaces formed from molybdenum.
Others have attempted to prevent degradation problems by treating the inner metal surfaces of the analytical apparatus with a passivating agent to mask or destroy active surface sites. For example, alkylchlorosilanes and other silanizing agents have been used to treat injectors, chromatographic columns, transfer lines and detectors in GC. See, e.g., U.S. Pat. No. 4,999,162 to Wells et al. Such treatments have been successful in deactivating metal surfaces and thus have prevented degradation of some species of analyte. Unfortunately, the materials used for such treatments have a sufficiently high vapor pressure to introduce organic materials in the gas phase within the volume of the ionization chamber that are ionized along with the analyte, producing a high chemical background in the mass spectrum.
In the field of atmospheric pressure ionization mass spectrometry it is common to separate ions or plasma from an atmospheric pressure region into a differentially pumped mass spectrometry system with an apparatus such as a skimmer installed between a first and second vacuum chamber. It is important for these devices to employ skimmers for separating compounds without interacting with them. In other words these devices should be designed to be inert to the compounds that pass through or contact their surfaces. Common skimmers are machined out of materials such as stainless steel. These devices are durable, expensive and are often damaged during abrasive cleaning.
Thus, there is a need to reduce deposition of contaminants, solvents and unwanted compounds onto mass spectrometer skimmers and their surfaces. There is also a need for skimmers that can be readily cleaned without damage to their surfaces.
The present invention relates to an apparatus and method for use with a mass spectrometer or MS system for ionizing a fluid sample. The skimmer of the present invention has a surface for reducing interaction with compounds and comprises a surface having an inorganic, conductive nitride compound. The nitride compound may be, for example, a titanium nitride or a mixed metal nitride such as an aluminum-titanium nitride or titanium-carbon-nitride.
The invention also provides a method of reducing interaction or deposition of a compound with a surface of a mass spectrometer skimmer, the method comprises applying a coating selected from the group consisting of nitrides of metals to the surface of the skimmer.